Slug Mucus

Snails and slugs can traverse vertical and inverted
terrain because
they crawl on slimy gel that is a complex fluid with non-Newtonian
properties. In the most basic terms, the slime acts like a solid glue
at rest, but liquifies when an adequate stress is a applied (a stress
exceeding the apparent yield stress). When the stress is removed, the
slime quickly re-solidifies. By exploiting this yield-heal property, a
snail can keep part of its foot stuck to the wall, while other parts
move forward. The picture shows the bottom of a Leopard
Slug, Limax maximus, during locomotion.